This invention relates to circuit breakers, having contacting metal operating, sensing, tripping and supporting members, functioning in an insulating liquid dielectric environment; and more particularly, to circuit breakers for distribution transformers, to control moderate power distribution on feeder circuits.
Circuit breakers contain a variety of sensing and tripping members such as shafts, springs, support pins, contact arms, bridging assemblies, etc. These parts are generally made of steel. Many of them contact each other and are aligned in close tolerances. Most prior art parts have been tin plated. This provided a protective film which could be welded, did not attract dust or affect the coefficient of friction of the steel, and could be applied in a non-degradable thin build, so that dimensional tolerances could be maintained in transformer oil. However, tin plating is very expensive.
One solution was thought to be the replacement of many of these coated steel members with plastic parts. However, holding the many dimensions with the associated close tolerances required extremely accurate molding. Also, the hot oil environment in which the circuit breaker must function is less than ideal for even the best plastics, and this led to loss of calibration during transformer processing and in service. Another solution was thought to be the simple application of an oil film to the parts. Such films, however, burn off in the welding and processing operations of constructing the breaker, exposing the steel to the atmosphere with resultant corrosion upon storage. This problem was particularly acute in southern high humidity or seacoast salt air environments.
An associated problem is the introduction of water into the operating transformer oil environment, and circulation of the water through the hot oil. The water could be introduced through seal leakage after a pressure buildup, or by decomposition of cellulose in the transformer coil assembly. Such circulating water could possibly attack inadequately coated metal breaker members, even in the oil environment.
Zinc phosphate coating compositions were found to flake and rust. Coatings of zinc dichromate and reducing agents were also found not to solve corrosion resistance problems in southern or salt laden atmospheres.
U.S. Pat. No. 2,768,104 treated steel strip with zinc phosphate, followed by a chromic acid treatment which could include the application of reducing agents such as phosporic acid, phenol, glycerine, potassium iodide, sugar or triethanolamine, to provide an anchoring lacquer for paint and resins. U.S. Pat. No. 3,706,604 teaches a similar process of single step conversion coating a phosphate treated metal surface with zinc dichromate and esterifying and/or reducing fixing agents, such as butyl alcohol, ethylene glycol, triethanolamine, hexamethylol-melamine, hydroquinone and dimethylol urea, to provide an adherent layer for subsequently applied paint or a corrosion resistant barrier layer. None of these patents dealt with coating circuit breaker parts which must resist high oil temperatures and be compatible with contacting oil in a circuit breaker environment.